Generally in the semiconductor processes, the process of Reactive Ion Etch (RIE) is used to etch imperceptible patterns on the silicon wafers. RIE includes a step of putting a wafer with photo-resist patterns into a reactive chamber with plasma. Plasma includes etching gas, which can be vertically decomposed in a RF (Radio Frequency) field. Therefore, the reactive ion in the etching plasma can leave the wafer surface accerlatively. The accelerated reactive ion includes the materials not covered by the photo-resist patterns on the wafer surface. Therefore, the volatile etching species will be produced. During the above etching process, one or multiple layers' materials or thin films can be removed. For example, the removed materials could be silicon oxide, polysilicon, silicon nitride, or metallic aluminum.
When a thin film not covered by the photo-resist pattern is etched, the volatile species will be merged into the plasma. Therefore, when the process of RIE comes to the end point, the etching gas will be decreased but the volatile etching species will be increased. These increased or decreased amount in the plasma could be traced back to decide whether the process of reactive ion etch goes to the end point or not. The FIG. 1 of U.S. Pat. No. 4,246,606 or 4,263,089 introduce a device with end point detecting method, wherein a photoelectric cell is utilized for transferring from the emission intensity of plasma to an electric current or voltage, and then the emission intensity of plasma is measured to decide every stage of the plasma etch process by the electric current or voltage.
There are many improvements of end point detecting method in the progress of semiconductor process. For example, in the U.S. Pat. No. 5,690,784, when a coated substrate is etched by plasma, there is a beam going through another monitor substrate with the same coating. The emission intensity of the beam will be changed during the process of etching. So we may decide whether the coated substrate comes to the end point of the process of REI or not by comparing the beam intensity with an expected value or a reference value. Further more, in the U.S. Pat. No. 5,160,576, the light produced by plasma discharge can go through a folded filter mirror to easily detect the end point by reducing the noises. And there still are other improvements of end point detecting method, just like U.S. Pat. Nos. 5,288,367, 4,936,967, 4,345,968, 4,687,539, 5,837,094, etc.
In the end point detecting method of reactive plasma etching, the process stability is very important. Usually there is a simple algorithm can decide the end point. This algorithm includes the delay time to bypass the unnecessary unstable duration produced by plasma at the radio frequency power-on. And then, the end point signal is averaged by the time during the stable duration. Finally, compare the average signal with the present signal level to decide the end of the process. At this moment, the emission of interested plasma species' single wavelength could be detected. There were several skills used to get better detecting efficiency of end point, which mainly enhance the mathematics to improve the average signal. For example, U.S. Pat. No. 5,565,114 calculates the summarized average intensity of plasma emission spectrum at first, and then calculates the difference or ratio of summarized average values to decide whether the etching process comes to the end point. However, this kind of skill can only be used in the stable processes or some unstable processes with guaranteed tolerance.
The much more complex methods include detecting two kinds of emission wavelengths concurrently. These methods are especially useful for detecting a weak emission in plasma. Usually the ratio between the two intensities of different wavelengths is used to detect the end point. However, due to the unstable characteristic of plasma, sensitive electrical instruments that surround the etching machine, and the sustaining changes of chamber's states, all these noises produced from every source will be pick up by the end point detecting device easily. Therefore, if the simple algorithm is used, then the computer system will misjudge an end point because a change of intensity from the RF power sudden surge or a polymer material. The other noises would come from the improper grounding, unstructured touching of chambers' components, the aging and lacking of electrical shielding components, contamination of chambers or wafers, or arcing in the plasma.
The instability of plasma is a very difficult problem to be overcome, but the other noise sources can be minimized by some mathematics methods, just like the time average. For the best result of time average, a long time is taken to calculate from the signal. This is impractical in the real process, and the improvement of manufacturing will be delayed by this solution very obviously.